In certain broadcast communications systems, such as, for example, satellite radio, in order to optimize the utilization of a fixed bandwidth, hierarchical modulation (“HM”) can be used to overlay data for new services on top of a legacy transmission. Such a scheme can be used, for example, to offer additional channels or services. For example, in the Sirius Satellite Radio, Inc. (“Sirius”) Satellite Digital Audio Radio Service (“SDARS”), video channels can be sent over existing audio channels via such an overlay modulation scheme, where the video signal is sent in a “Layer 2” or overlay modulation layer, on top of an existing audio service, known as the “legacy” signal. There are many approaches to hierarchical modulation, each utilizing a further modulation of a transmitted legacy bit or symbol as to amplitude, phase or a combination of the two. For example, hierarchical modulation can involve the perturbation of original legacy Quadrature Phase Shift Keying (“QPSK”) symbol constellation points, which can, for example, carry audio and data traffic, to convey additional information, such as, for example, video. Thus, for example, such an overlay modulation scheme can carry video data fully independently of the legacy data (original QPSK symbol) carrying a variety of audio channels. For example, Sirius' Backseat TV™ service uses an overlay modulation technique to send video on top of its existing legacy audio channels.
Existing satellite broadcast communication systems, such as, for example, Sirius' SDARS, can employ two forms of modulation to convey information, single carrier QPSK and multi-carrier differential COFDM.
QPSK is a modulation technique that allows for the transmission of digital information across an analog channel. In QPSK, data bits are grouped into pairs with each pair represented by a particular waveform, commonly referred to as a symbol. There are four possible combinations of data bits in a pair, and a unique symbol is required for each possible combination of data bits in a pair. QPSK creates four different symbols, one for each pair, by changing the I gain and Q gain for the cosine and sine modulators. The symbol is then sent across an analog channel after modulating a single carrier. A receiver can demodulate the signal and look at the recovered symbol to determine which combination of data bits in a pair was sent. COFDM is a modulation technique that can distribute a single digital signal across one thousand (1,000) or more signal carriers simultaneously. Coded data is modulated and inserted into orthogonal carriers in the frequency domain. Because signals are sent at right angles to each other, the signals do not interfere with one another. In general, the term “multi-path effects” refers to the scattering of a signal due to obstructions such as canyons, buildings, etc., that can cause a signal to take two or more paths to reach its final destination. COFDM is highly resistant to multi-path effects because it uses multiple carriers to transmit the same signal. Thus, techniques to implement hierarchical modulation over legacy COFDM transmissions are a desideratum in the art.